The invention relates to an optical fire-detector in which a radiation-emitting means is arranged to emit a beam of radiation and which has modulator means for the modulation of the beam of radiation in a phase-inverted relationship within a first and a second wavelength band and a radiation-detecting means is arranged to receive the beam of radiation after it this has passed through an intermediate air medium and includes first means for an individual measurement of the intensity in the two wavelength bands and second means for the detection of such variations in the measured intensities which are representative for a fire.
An optical fire-detector of the above defined type is described in the Swedish Patent application No. 7604502-0 where the mutually phase inverted modulation within the first and the second wavelength band is for the purpose of enabling the individual measurement of the intensity in the two wavelength bands to be carried out by means of a single radiation-sensitive element. The fire detector can obtain a good discrimination against flicker generated by the surrounding electrical illumination by a method which is described in the Swedish Pat. No. 7310965-4. According to such method the beam of radiation is emitted in the form of a series of narrow high-power pulses, the radiation-detecting means being arranged to be frequency-selective for the rise time of the pulses.
One drawback with this known method is, however, that the fire detector achieves the desired discrimination against flicker generated by the surrounding electrical illumination only if the radiation detector as well as the radiation-emitting means have a short rise time of the order of .mu.s. Thus, this method does not enable an efficient use of such radiation-sensitive elements in which a high sensitivity is achieved at the cost of a long rise time of the order of 100 .mu.s.
The optical fire detector according to the invention achieves a good discrimination against flicker generated by surrounding electrical illumination without requiring a short rise time neither at the radiation detector nor at the radiation emitting means and enables an improved discrimination against such flicker which is generated when mechanical vibrations for example caused by heavy street traffic vary the outgoing direction of the beam of radiation from the radiation-emitting means.